As for a recording medium in which a plurality of color developing layers having different threshold values of color developing temperatures, respectively, are laminated, when a prescribed region on a surface thereof is irradiated with laser and the color developing layer is heated, prescribed information can be recorded therein.
An information recording medium to be used such as an ID card and an IC card is configured by laminating a plurality of color developing layers having different threshold values of color developing temperatures, respectively. When, of two main surfaces of the information recording medium, one surface which is to be irradiated with laser is called a front surface, a prescribed region on the front surface is irradiated with laser and the color developing layer is heated, and thereby prescribed information can be recorded. In the information recording medium, in a case where information is printed in the form of a color image, three color developing layers of different colors are laminated. For example, when printing of the color image is realized by the three primary colors (magenta, yellow, cyan), three color developing layers which respectively develop colors at temperatures of about 100° C., about 150° C., about 200° C. are laminated inside the information recording medium, in the order distant from the front surface. The color developing layer having the color developing temperature of about 100° C. is a cyan color developing layer, the color developing layer of about 150° C. is a magenta color developing layer, and the color developing layer of about 200° C. is a yellow color developing layer.
Spacer layers having heat insulating property are arranged among the three color developing layers so as to delay heat transfer. That is, “a cyan color developing layer, a spacer layer, a magenta color developing layer, a spacer layer, a yellow color developing layer” are laminated in the information recording medium, in the order distant from the front surface. When the front surface of the information recording medium is irradiated with laser to apply heat to the color developing layers, the color developing layer to be developed can be selectively developed by varying how to apply heat for each color developing layer to be developed. Namely, that the heat generated in the vicinity of the front surface with laser irradiation is transferred to the respective layers and the temperatures of the respective layers are changed is controlled by how to apply heat with laser, that is, a laser irradiation condition, and thereby it is possible to make the respective color developing layers selectively develop colors.
For example, the spacer layer is arranged between the yellow color developing layer and the magenta color developing layer so that a temperature of the magenta color developing layer does not rise up to 150° C. when the yellow color developing layer is made at a temperature not less than 200° C. to develop a color. The thick spacer layer is arranged between the magenta color developing layer and the cyan color developing layer so that the temperature of the cyan color developing layer does not rise up to 100° C. when the yellow color developing layer is made at a temperature not less than 200° C. to develop a color, or the magenta color developing layer is made at a temperature not less than 150° C. to develop a color. It is necessary to make the spacer layer between the magenta color developing layer and the cyan color developing layer thicker compared with the other spacer layer because the heat conductivity easily becomes larger at a low temperature compared with at a high temperature. That is, it is necessary that the thickness of the spacer layer between the two color developing layers located at positions distant from the front surface is made thicker than the thickness of the other spacer layer.
Since the heat is transferred not only in the depth direction, but also in the direction along the front surface, when the cyan color developing layer located at the most distant position from the front surface among the three color developing layers is made to develop a color, heat expansion becomes large due to the thick spacer layer, and thereby it is difficult to perform high resolution printing. In addition, in order to make the cyan color developing layer selectively develop a color, it is necessary to keep the cyan color developing layer at a temperature lower than 150° C. for a long time so as to transfer heat to the cyan color developing layer through the thick spacer layer. For this reason, the printing time easily becomes long. In addition, since the color developing temperature of the cyan color developing layer is about 100° C., in a case in which the information recording medium is heated at about 100° C. when in use, unintended color development may occur, and thereby the information recording medium has the tendency for heat resistance to be low. That is, it is desired to improve the property of the information recording medium.